Automatic Gain Controllers (AGCs) are typically used for two reasons: 1) in a receiver, to prevent saturation of one or more stages which is caused by exceeding the dynamic range of a stage by a large input signals, and 2) to keep the average output level of some type of acoustical means (e.g., a speaker) nearly constant in the presence of a signal whose power level is varying. Techniques for achieving the above are well known for both analog and digital construction of radio frequency (RF) receivers. Known techniques, however, result in a receiver with a significant reduction in noise figure, which results in loss in sensitivity, and/or a receiver with a significant loss in dynamic range because the AGC reduces the linearity of the stage that is gain controlled.
Many advantages of using digital signal processing as applied to RF receivers are well known. It is also well known that a major limitation in the application of digital signal processing techniques in RF receivers is the limited dynamic range of the analog-to-digital converter (ADC) and/or the large noise floor of the ADC. One notable advantage of using digital signal processing is the ability to have well-controlled filters, and the ability to easily change filter characteristics for different types of communication systems. If an RF receiver functions within several different types of communication systems, any analog selectivity before the ADC must have sufficient bandwidth to support the signal with the largest bandwidth that the system must support. Hence, the RF receiver system, and in particular the ADC, must have a dynamic range large enough to allow the receiver to handle multiple signals simultaneously (e.g., a desired signal and an adjacent channel signal). At present, an ADC is not known that has a dynamic range large enough to support the needs of a land mobile communication system without the use of some type of AGC. Ideally, the type of AGC used should result in minimum impairment to noise figure and dynamic range of the system.
A particular type of ADC that is well-suited for narrow band radio communication systems is a sigma delta converter. Sigma delta converters are capable of being gain controlled. A known AGC apparatus is a digital feed-forward AGC that follows the ADC, where the ADC has sufficient dynamic range to cover the system's needs.
Applying AGC techniques to digital receivers as well as control of front end gain and digital gain is well known. The control of front end gain overcomes the problem of insufficient dynamic range of the ADC. Nevertheless, known techniques using analog gain reduction result in a reduction of the linearity of the receiver's front end analog circuits. Thus, dynamic range may be reduced due to intermodulation when multiple signals are present.
Various AGC techniques exist that are used with both analog and digital receivers. These techniques result in either a limited front end gain or requiring that the ADC be capable of supporting the entire dynamic range, again resulting in potential dynamic range loss due to receiver front end linearity reduction.
Accordingly, there is a need for an AGC or similar technique in a digital setting which has good noise figure, good receiver sensitivity, limited noise contribution from the AGC, and no significant loss in dynamic range.